itree.[ch]: Add sanity checks, comments, and minor tweaks

* src/alloc.c (mark_overlay): Add sanity check.

* src/buffer.c (next_overlay_change, previous_overlay_change):
Tweak code to keep the same vars for the bounds.

* src/itree.c (interval_tree_clear, interval_tree_insert)
(interval_tree_remove, interval_tree_insert_fix, interval_tree_remove_fix):
Adjust to the `color` -> `red` change.
(interval_tree_clear): Prefer `true/false` for booleans.
(interval_generator_create): Use an actual `interval_tree_order` value
rather than 0.
(interval_generator_next): Simplify a tiny bit.  Add comment.
(interval_generator_narrow): Add sanity check.

* src/itree.h (struct interval_node): Replace `color` field with
boolean `red` field.
(enum interval_tree_order): Remove unused `ITREE_DEFLT_ORDER` value.

* src/pdumper.c (dump_interval_node): Adjust to the
`color` -> `red` change.

1 files changed, 2 insertions, 2 deletions

diff --git a/src/buffer.c b/src/buffer.c
index 2b1997fc8be..879e14be960 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2992,7 +2992,7 @@ next_overlay_change (ptrdiff_t pos)
   ptrdiff_t next = ZV;
   struct interval_node *node;
 
-  buffer_overlay_iter_start (current_buffer, pos, ZV, ITREE_ASCENDING);
+  buffer_overlay_iter_start (current_buffer, pos, next, ITREE_ASCENDING);
   while ((node = buffer_overlay_iter_next (current_buffer)))
     {
       if (node->begin > pos)
@@ -3020,7 +3020,7 @@ previous_overlay_change (ptrdiff_t pos)
   struct interval_node *node;
   ptrdiff_t prev = BEGV;
 
-  buffer_overlay_iter_start (current_buffer, BEGV, pos, ITREE_DESCENDING);
+  buffer_overlay_iter_start (current_buffer, prev, pos, ITREE_DESCENDING);
   while ((node = buffer_overlay_iter_next (current_buffer)))
     {
       if (node->end < pos)